DutyCycleEncoder Expected Zero improvements

[sciencewhiz]

Is your feature request related to a problem? Please describe.
Setting the DutyCycleEncoder Expected Zero is a multi-step process. First you must run the code with the range set, but with Expected Zero set to 0 to figure out what the expected zero should be, then you must put that value in the constructor. Then if you later need to change the expected zero (for example if a mechanism is reinstalled), you need to read the current value, and add the current Expected Zero value, to figure out what the new Expected Zero value should be, or else recompile the code with Expected Zero as 0, and find the new value to avoid the mental math.

Describe the solution you'd like
A method to set the Expected Zero at runtime, and a method to get the encoder value without the Expected Zero applied (but with the range scaling).